The GPS is a satellite-based navigation system. A GPS receiver measures the time offset between a Pseudo Random Noise (PRN) code received from a GPS satellite and a locally generated PRN code. The GPS receiver uses the time offset (referred to as a pseudorange measurement) from at least four satellites along with satellite position information broadcast by the satellites (referred to as ephemeris) to determine the receiver's location. The pseudorange measurement and satellite ephemeris are intentionally degraded by the U.S. Government using a Department of Defense (DoD) classified technique called Selective Availability (SA). An authorized user can enter a key supplied by the National Security Agency into the receiver. The receiver will use this key to remove the effects of SA from the pseudorange measurement and satellite ephemeris, thereby accessing what is referred to as the GPS Precise Positioning Service (PPS).
A GPS satellite also broadcasts, in encrypted form, Wide Area GPS Enhancement (WAGE) corrections. An authorized GPS receiver decrypts the WAGE corrections using the same key as used for SA and applies them to the pseudorange measurements to compensate for unintended system errors. WAGE corrections are not required to obtain PPS accuracy, but do allow the GPS receiver to obtain even better accuracy than PPS alone. Since WAGE corrections may be treated as just another component of SA compensation, references to SA in this disclosure should be understood to mean both with and without WAGE.
The DoD has classified SA correction components as secret. A keyed receiver must compute the SA correction components and apply them to the ephemeris and pseudorange measurements. The DoD has also classified as secret both satellite position derived from corrected ephemeris and pseudorange measurements corrected for satellite clock related SA components as secret. The DoD controls the security boundaries for all PPS-capable GPS receivers. A GPS receiver's security boundary includes all hardware and software that access classified data. The DoD requires that security reviews be held to ensure that the boundary is defined properly, that classified data within the boundary is handled properly, and that the hardware and software in the boundary are developed using the appropriate procedures. These special reviews and procedures are expensive, and the more receiver hardware/software inside the security boundary, the more involved the process.
The process becomes even more complicated when a host application needs to process pseudorange measurements outside the GPS receiver. This entails definition of a data interface between receiver and host that shall be referred to herein as a "GPS measurement interface." If a GPS receiver makes corrected satellite position and pseudorange measurements available over such an interface, the interface becomes classified secret and the security boundary of the receiver extends to include parts of the host application. This further increases cost since the host must now participate in the DoD security review process. A GPS measurement interface whose purpose is to enable processing of pseudorange measurements from a keyed receiver with PPS accuracy shall be referred to herein as a "PPS measurement interface".